Like all root vegetables, much of onion growth is hidden below the soil surface.
This article will unearth the truth about these versatile bulbs, explaining the best growing conditions for onions, the onion life cycle, and the onion growing stages.
Plus, we’ll also debunk some myths about spring onions and scallions.
Onion Growing Conditions
Likely native to southeast Asia, onions now grow in temperate zones worldwide.
In the United States, onions grow year-round. California produces the most onions, over 25% of the nation’s total onion production. According to the USDA, the Golden State grew an estimated 1,902,600,000 pounds of onions in 2021.
Onions are a hardy vegetable. But seedlings should only go into the soil once temperatures consistently remain above 28° F.
Once planted, onions need at least six hours of full sun each day and prefer temperatures ranging from 55 – 75° F. Warmer temperatures tend to produce sweeter and milder onions.
This bulb vegetable has shallow root systems, making it easy to grow onions in both pots and the ground. However, onion seeds will produce smaller, stunted bulbs without adequate space between plants.
These shorter roots require well-drained soil that is rich in organic matter. Onions will not grow well (if at all) in clay soils, which drain slowly and can become waterlogged.
Onions prefer frequent watering but not soggy soil. Overwatered onions will begin to rot.
The best time to plant an onion crop will depend on a few factors: onion variety, regional growing season, available sunlight, and whether you’re growing the vegetable by seed or bulb.
Let’s look at the onion life cycle in a little more detail.
Life Cycle of an Onion
The onion is a biennial plant, meaning it has a two-year lifecycle.
During the first growing season, the onion concentrates its energy on growing green vegetation and storing energy in the bulb. Therefore, farmers tend to harvest onions at the end of the first growing season, as this is when the bulb is at its biggest size.
If left to grow into a second season, the onion will begin the reproductive process. Bulb size reduces as the plant redirects reserved energy into sprouting and growing a flower.
This process is known as “going to seed” or “bolting”. Reaching this point of onion growth is only desirable for farmers who intend to harvest onions for the seeds rather than the bulbs.
Remember: the bulb is where the onion stores energy, so draining this reserve causes the bulb to shrivel.
Growing onions from seed reduces the risk of bolting. But it requires planning.
Before planting onion seeds in the ground, you will need to grow seedlings for six to eight weeks in a greenhouse.
Growing onions from tiny bulbs cuts the growing time in half. These bulbs, also known as onion sets, are onions harvested and stored during the previous year. Since these onions are in their second growing season, they are more likely to bolt.
It is also possible to grow onions from transplants. Onion transplants are juvenile onion bulbs, fresh from this year’s season. This youthfulness means they are less likely to bolt than onions grown from sets.
Onion transplants produce the biggest bulbs in the shortest amount of time with the least amount of work — making them a handy option for beginner gardeners.
Whichever growing method you choose, remember the vegetable’s one-to-one propagation. One bulb will produce one onion. Likewise, one onion will grow from one onion seed.
Onion Growing Time Lapse
If you’re more of a visual person like me, check out this onion growing time lapse video:
10 Onion Growing Stages of the Common Onion
1.) First Is Planting
Different planting methods exist for growing onions, such as from seeds, onion sets, or mature plants. Onions are cool-season vegetables that can be planted in early spring or fall, depending on your location.
For spring planting, wait until the soil temperature reaches a consistent 28℉ before planting onion sets or transplanting seedlings into your garden beds. This typically happens in late March or early April, just before the last frost date of the season.
If you’re starting from seed, give yourself a head start by planting them 8 to 10 weeks ahead of time.
If you’re more of a fall gardener, you’ll want to aim to plant your onion sets in August or September so they have enough time to grow before the temperatures drop. As the cool weather sets in, your onions will go dormant, only to resume growth in the spring when things start to warm up again.
Yarrow is a great companion plant to onions and can be planted together.
Planting from seeds is the simplest way to grow onions, while onion sets are easier to grow than seeds.
You can also use the bottom part of a mature onion plant to grow new sprouts. When planting, it’s important to wait for the right temperature and keep the soil moist.
Check out this video for more tips on planting onions:
Germination is the process during which an onion seed develops into a plant.
After a period of dormancy, several combined factors can initiate seed germination: water absorption, light exposure, temperature change, oxygen availability, and the passage of time.
When the seed’s embryo absorbs water, the cells inside the seed expand. This process, known as imbibition, increases the seed’s respiration and metabolic processes. The organelles in the embryo experience structural changes.
What does that look like to the naked eye?
Roots develop downward, anchoring into the soil. These roots hold the seed in place and absorb nutrients and moisture.
This process depends on temperature, humidity, and soil type. Well-drained soil rich in nutrients is optimal for growing onions, while clay soil should be avoided. If the growing conditions are not optimal, this onion growing stage may take a few weeks longer.
The lime green sprout develops upwards, pushing through the soil’s surface. Above the soil surface, this shoot allows the plant to perform photosynthesis. Photosynthesis produces food for the onion.
From start to finish, onion germination takes about ten days. Temperature, humidity, soil quality, and nutrient availability all impact germination time.
3.) Vegetative Growth / Sprouting
The seedling rapidly forms vegetative growth over two weeks, during which proper moisture, warmth, and fertilization are crucial.
During photosynthesis, the seedling converts sunlight into energy. The onion plant uses this energy to grow green vegetation.
These leaves look like smaller, fleshier versions of those found on a mature onion. Although still tiny, these ever-elongating leaves increase the plant’s ability to perform photosynthesis.
Once the seedling has produced several sets of mature leaves, the onion has successfully established in the soil.
Note: If you started your onion plant/s indoors, it should be transplanted into the garden at this stage. But wait! Don’t forget to apply some mulch around the base of your precious plants. Mulch is like a cozy blanket for your onions, helping them retain moisture and keeping pesky weeds at bay.
4.) True Leaves Start To Develop
Picture this: your onion sprouts are now starting to look like real plants! They’ve grown a smaller version of dark green leaves, signaling the formation of true leaves.
At this point, your plant is on its own, relying solely on photosynthesis and the ability to produce glucose for energy.
As more true leaves develop, your sprouts will start to resemble leeks, and small onions will soon emerge.
Once the stalks have reached their full height and have that sturdy, dark green look, your onions are now considered scallions, green onions, or bunching onions – but their botanical name, Allium Cepa, remains the same throughout their growth.
Letting your onion plant finish this stage quickly is vital so bulb development can begin. The sooner the bulb starts forming, the larger your onions will be! Remember to watch your plant’s nutrient and moisture levels.
5.) Bulb Formation
After developing at least four robust leaves, the onion plant begins to direct energy toward bulb formation. Located at the point where the stem meets the root system, the bulb is the edible part of the common onion.
Vegetative growth and bulb formation occur simultaneously until the plant has produced eight to ten leaves.
At this point, vegetative growth stops. However, bulb formation continues for a period of weeks to months.
The plant transports food from the above-ground leaves to the scales below the surface. As they receive this energy, the scales swell. The scales eventually form rings around the bulb, providing a central storage tissue.
Length Of Time & Varieties Of Onions
The length of time for the bulbs to develop usually takes anywhere from 70 to 120 days. However, this time can vary depending on the onion variety and the growing conditions.
Different onion varieties have different day-length requirements for bulb formation. Short-day onions, which need 10 to 12 hours of daylight, are ideal for warmer regions with mild winters, while long-day onions, requiring 14 to 16 hours of daylight, are better suited for cooler regions with longer days.
Day-neutral onions need 12 to 14 hours of daylight and can be grown in most regions.
Long-day onions typically produce the largest bulbs, making them the most commonly sold variety. However, if you’re growing onions primarily for their green tops, then short-day onions are the way to go.
So sit back and enjoy watching your onion plant grow and develop those delicious bulbs, but remember, patience is key!
6.) Bulb Maturation
As the bulb matures, it eventually begins to emerge from the ground. Next, the above-ground leaves turn brown until this vegetation eventually droops toward the ground.
If grown from seed, onion maturation takes about four months. Growing onions from sets decreases this time to just under three months.
Also, look to see if the leaves of your onion plant are folding down; it indicates that the nutrients have moved from the top of the leaves to the bottom, leading to the swelling of onion bulbs. This is a sign that the onion is ready to be harvested.
7.) Harvesting or Reproduction
Determining when bulbs have reached full maturation depends on the regional growing season and the variety of onions you intend to harvest. (More on this topic in the next section.)
When harvested too early, onions will taste bitter. But harvesting onions too late increases the risk of rotting. It can be tricky to judge when an onion is ready to harvest, so pay attention to how the top leaves appear.
Brown, wilted top leaves and an exposed bulb indicate that the onion is ready for harvest.
Harvesting onions is fairly straightforward. Use a digging fork to loosen the soil around the onions, then carefully pull out the onions. Or use a spade or shovel to dig out the onions.
After removing onions from the soil, you need to cure them for about 20 to 25 days. This process allows the onion to keep its shape.
To cure your harvest, hang onions in a dry place for four to six weeks. After curing, store the onions in a cool, dry place. They will last up to several months.
Remember, onions are a biennial plant. So if you don’t harvest them at the end of the first growing season, they will continue their growth cycle in the following spring.
Since the root systems have already been established, second-year onions will focus their energy on growing a flowery stalk for reproductive purposes. This process is known as bolting and causes onions to shrivel as the plant sends energy above ground.
If left to continue this end-of-life cycle, onion stalks will produce buds with a white or purple flower at the top of the plant.
8.) Flowering Stalks Begin
As the seasons progress and temperatures rise, the onion plant will persist in its growth. Eventually, the vegetative growth will cease and the plant will begin to sprout its flowering stalks.
You’ll notice small buds emerging at the top of these stalks, which will eventually blossom into delicate flowers.
The hue of these blooms may vary, depending on the specific onion variety, but typically they’ll be white, green, or purple. This floral display signals the end of the onion’s lifespan.
9.) Production Of Seeds / Pollination
Bees, butterflies, and birds pollinate these flower heads, which contain the onion’s seeds. Once seeds appear, the plant has completed its life cycle.
When the onion plant reaches the seed production stage, it will utilize all the nutrients stored in the bulb to generate seeds. If you were to harvest the onion at this point, the resulting bulbs would likely be dry and unappetizing.
However, there’s still a silver lining! You can collect and save the seeds to plant in your garden during the next growing season. This is especially beneficial if you’ve had a bountiful harvest and the specific variety of onion is particularly pleasing to your palate.
10.) Senescence: the Last Of the Onions Growth Stages
Once the plant has completed its seed production, it has reached its final onion growth cycle stage. At this point, the plant will gradually deteriorate and ultimately perish, having expended all of its energy reserves.
We like to leave a few onions in the ground over the winter so that I can utilize their seeds for future plantings. However, this is a matter of personal preference, and you can choose to handle your onions however you see fit.
Just be aware that if you do decide to use last year’s seeds, you won’t be able to start your new onion plants indoors.
Spring Onions & Green Onions
There’s a common misconception that both spring onions and green onions are both immature versions of common onions. Spring onions and green onions certainly look very similar, but they are not the same vegetable.
Spring onions are common onions that the grower has harvested early. Since they are the same species, the growing stages of spring onions and common are exactly the same.
The only difference is that growers harvest spring onions before the bulbs reach maturation. Typically, growers harvest spring onions when the bulb has reached about an inch in diameter. At this point, the top vegetation is still firm and green, so these leaves are often left attached.
Green onions, also known as scallions, are not the same species as common onions. Less than half an inch in diameter, the underdeveloped bulb looks more like a thick white stalk.
Although many commercial growers pass off immature common onions as scallions, true green onions come from Chinese onion or Welsh onion plants.
Onion Growing Tips
Now that you’ve learned about the different onion growing stages, here are five growing tips to ensure success.
- When preparing the garden bed, make sure it’s fertile and has good drainage. Add compost, well-rotted manure or a small handful of fertilizer.
- Sow onion seeds shallowly and cover lightly, keeping the soil moist until sprouting starts. Sow a generous amount of seeds to allow for poor germination. Transplant seedlings from areas that have germinated well to areas that are sparse. Mulch the bed to reduce weeds.
- Sow onion seeds at the right time for your climate. In subtropical climates, sow towards the end of autumn. In warmer climates, sow through the driest part of the year. In cold climates, sow through spring.
- Grow the right variety of onion for your climate and match the variety to the daylight hours at the time of year you are growing them. Fast or early maturing varieties are preferred for winter planting.
- Crowd the onions in the garden bed to maximize space. This helps to prevent weeds and encourages the onions to grow taller instead of wider.
Common Problems to Avoid
It’s true that growing root vegetables can be challenging, but with proper care and attention, you can minimize the risk of crop damage and failure. Here are some common issues that may arise during onion growing stages, along with tips on how to deal with them:
If your onion seedlings are growing slower than they should, it could be due to poor soil conditions or lack of nutrients. Ensure that your soil is well-draining, loose and has a pH of 6.0 to 6.8. Additionally, apply a balanced fertilizer to provide essential nutrients to the plants.
If your onion leaves are turning yellow, it could be due to nutrient deficiency, lack of water, or pests. Make sure you water your onions regularly and feed them with a fertilizer that contains nitrogen, phosphorus, and potassium. Also, keep an eye out for onion maggots or thrips, which can cause significant damage to the leaves.
If your onion plants are wilting, it could be due to either overwatering or underwatering. Make sure you water your plants appropriately, based on the soil moisture level and the weather conditions.
If your onion bulbs are not forming properly, it could be due to inconsistent soil moisture levels or soil compaction. Make sure you water your onions regularly and loosen the soil to allow room for the bulbs to grow.
Pests and Diseases
Onion maggots, thrips, and fungal diseases like downy mildew and white rot can cause significant damage to your onion crops. To prevent these issues, practice good crop rotation, maintain good hygiene by removing infected plants promptly, and use natural or chemical pesticides as needed.
FAQ About Stages of Onion Growth
How long do onions take to grow?
This depends on many factors, such as the type of onion, climate, and growing conditions. But, generally, onions take about 3-5 months to mature from seed to harvest. However, some fast-maturing varieties can be ready for harvest in as little as 60 days, while others can take up to 7-8 months. So, consider the type of onion and the climate when deciding when to plant them to ensure a successful harvest.
How do you know when onions are done growing?
Onions are done growing and ready to harvest when the foliage starts to yellow and fall over. This typically occurs in late summer or early fall, about 100-175 days after planting, depending on the variety and growing conditions.
Davin is a jack-of-all-trades but has professional training and experience in various home and garden subjects. He leans on other experts when needed and edits and fact-checks all articles.